It has been known that a stereo demodulation system for an FM stereo broadcast receiver incurs the problem of the generation of beat noise. When an FM stereo broadcast receiver which is receiving a broadcast wave transmitted by a desired broadcast station receives also another broadcast wave transmitted by an undesired broadcast station, the frequency difference betwen the two stations being, for example, 200 kHz and the maximum frequency deviation being 75 kHz, the frequency spectrum of the output of the FM detector can include an additional spectral range (ASR) illustrated as the hatched portion in the frequency spectrum of FIG. 4. When stereo demodulation is effected by using a signal having the frequency spectrum of FIG. 4 and the signal S(38) of 38 kHz, undesired components which cause unpleasant noise such as birdy-noise are produced due to the beats between the additional spectral range ASR (FIG. 4) and the odd higher harmonic frequency signal such as the third harmonic frequency signal S(114), and the fifth harmonic frequency signal S(190). If the frequency of the subcarrier signal is 38 kHz, the third and the fifth harmonic frequencies are 114 kHz and 190 kHz, respectively. Among such beat noises, the beat noise due to the third harmonic frequency S(114) is the first significant one and the beat noise due to the fifth harmonic S(190) is the second significant one.
A known prior art system is illustrated in FIG. 1 in which such beat noise is reduced by connecting a low pass filter 14, such as a beat-cut filter or an anti-birdy-noise filter between a receiving and detecting circuit 12 and a buffer amplifier 15. The FM stereo broadcast receiver of FIG. 1 comprises an antenna 11, the RF, IF amplification and FM detection stage 12, the low pass filter 14, a buffer amplifier 15, a capacitor 3, a phase comparator 4, a frequency divider 5, a low pass filter 61, a DC amplifier 62, a voltage controlled oscillator 7, a frequency divider 800, and a switch and decoder circuit 2. The decoder circuit 2 produces the demodulated left channel output (L) at the terminal 201 and right channel output (R) at the terminal 202. However, the system of FIG. 1 has a problem that the added low pass filter 14 exerts an undesirable effect on the ability of separating left and right channels of the stereo demodulation system and the frequency response characteristic of the demodulated output signal.
Improved prior art systems are illustrated in FIGS. 2 and 3 in which one signal, which is produced by switching the composite signal by a signal of a subcarrier frequency, and the other signal, which is produced by switching the composite signal by a signal of the third harmonic frequency of the subcarrier, are obtained, and addition and subtraction processes are effected between said one and the other signals, so that the beat noise is reduced. Such improved prior systems have been proposed by the inventors of the present invention in Japanese Patent Application No. 54-14230 (corresponding to U.S. Ser. No. 118,974, now U.S. Pat. No. 4,334,125).
The systems of FIGS. 2 and 3 comprise frequency dividing means 800 which includes frequency dividing flip-flop circuits 801, 802, 803, 804, a first switch and decoder circuit 21, a second switch and decoder circuit 22, and an addition/subtraction circuit 23. A first switching signal SS.sub.1 produced from the frequency divinding means 800 is supplied to the first switch and decoder circuit 21, while a second switching signal SS.sub.2 produced from the frequency dividing means 800 is supplied to the second switch and decoder circuit 22.
Since the principle of generation of the beat noise and the principle of the cancellation of the beat noise are described in the above referred preceding application, only a rough illustration of the analysis of the generation of the beat noise and the cancellation of the beat noise is shown in FIG. 4. Effecting the switching by the signal S(38) of 38 kHz, "L-R" signal is obtained in the demodulated signal. At the same time, a beat component BC(+) is formed by the multiplication of the additional spectral range ASR by the signal S(114) of 114 kHz. Also, effecting the switching by the signal S'(114) of 114 kHz which is opposite the signal S(114) a beat component BC(-) is formed by the multiplication of the additional spectral range ASR by the signal S'(114). Since the beat component BC(-) is of the same amount as that of the beat component BC(+) and has the opposite sign to that of the beat component BC(+), the beat components BC(+) and BC(-) are cancelled when the beat components are introduced into the addition/subtraction circuit 23.
However, in the circuit of, for example, FIG. 3, it is difficult to perfectly maintain the fixed phase relationship between the first and second switching signals SS.sub.1, SS.sub.2. The output signal S.sub.4 of the flip-flop circuit 801 can have the wave form either of S.sub.4 (A) or of S.sub.4 (B) as illustrated in FIG. 5. That is, it is uncertain which signal whether a signal having wave form S.sub.4 (A) or a signal having wave form S.sub.4 (B) is produced as the output signal S.sub.4 when the signal S.sub.1 is applied to the input terminal of the flip-flop circuit 801. This uncertainty is caused by the inherent characteristic of a flip-flop circuit.
Assuming that the beat cancellation is carried out only when the relationship between the wave forms S.sub.1 and S.sub.4 is such that when the signal S.sub.1 arises the signal S.sub.4 falls, the signal S.sub.4 having the wave form S.sub.4 (B), achieves the cancellation of beat noise, while the signal S.sub.4, having the wave form S.sub.4 (A), cannot cancel beat noise but even doubles the beat noise. This constitutes problems in the system of FIG. 3.